A transition from fluorescent and other forms of traditional lighting to Light Emission Diode (LED) illumination is occurring in various environments including retail outlets, office buildings, warehouses, hospitals, and private homes. LED illumination may provide the benefits of low power consumption, low running cost, long life, and high color rendering effect among other desirable features. Some nations are now moving to ban further manufacture of conventional light bulbs for environmental reasons.
Bar code readers are commonly used in retail environments, including convenience stores, supermarkets and the like. Generally, a laser-scanned barcode reader operates by sweeping a laser beam, commonly having a 650 nm wavelength, over a bar code and receiving light energy reflected from the bar code, which is processed to generate a bar code signal. In a typical application, a laser beam using a 100 Hz scan rate will produce a signal having a frequency range of 30 kHz (kilohertz) to 200 kHz, depending on the resolution of the bar code and the read distance (the distance from the bar code to the bar-code reader).
To suppress power consumption, LED bulbs are generally driven at a frequency within a range of about 30 kHz to 100 kHz, a range which overlaps with the frequencies of many bar code signals. It would be hard for a bar code reader to distinguish light energy from ambient light from light energy from a bar code signal if the frequency ranges of the two signal types overlap. To eliminate interference of the ambient light with bar code readers, U.S. Pat. No. 6,811,087, which is incorporated by reference herein, discloses a technique to scan a bar code using a pulsed laser at a frequency of 2 MHz (megahertz) and using a synchronous detector to detect this frequency and preferably no other frequencies. This technique significantly removes ambient light having a constant intensity (such as sunlight) and light energy from high frequency L.E.D. illumination. However, where there are ambient light frequency components in common with a bar code signal, the decoder within the bar code reader could misread ambient light as being part of a bar code signal, which could lead to a signal reading failure.
Moreover, other possible sources of noise may be present in bar code reading environments as discussed in the following. Laser-scanned bar code readers commonly have exit windows made of glass or plastic (i.e., polycarbonate, Polymethyl methacrylate material) to protect the sensitive parts inside the reader housing. Although coated with an anti-reflective film, dirt or a finger-print on the exit window would present an optical obstruction resulting in significant back-scatter light being directed toward the photo sensor. The back-scattering of light would be more severe in a retro-reflective type barcode reader, in which the outgoing laser beam and the collected light beam received by the reader share the same optical path. Whereas the signal intensity from a bar code at a distance of 300 to 500 mm has a magnitude of about 0.1 uW (microwatts), the back scatter light could reach a magnitude of 1 uW, which is ten times the magnitude of the bar code signal. The above-described situation may thus lead to an inability of the bar code reader to accurately read a bar code. Accordingly, suitable amplification of the bar code signal is desirable.
Existing preamplifier circuits have amplifier controllers that provide feedback resistance that is controllable based on the magnitude of the output signal. When the output magnitude causes the amplified signal to reach the saturation point of the circuit, the resistance used as part of the amplification circuit is decreased to a smaller value by adding resistor in parallel to the feedback resistance, to lower the output to a level below the saturation level. However, the resistance remains at a larger value if the output is safely below the circuit saturation level, in order to maintain a high signal-to-noise ratio. The feedback resistance can be altered such that the output always reaches a predetermined maximum value.
The existing art also discloses a low pass filter coupled to an amplifier as described above for transmitting therethrough a low-frequency component of the voltage signal amplified by the preamplifier. A thresholding function may be implemented such that if the output signal of the low pass filter is lower than a predetermined level, the output is brought to a zero level. A function may be implemented to linearly increase the magnitude of the output signal when the output signal is equal to or higher than a selected predetermined level.
Using an existing approach, two or more different feedback resistance values are switched depending on scanned signal levels. When the feedback resistance is switched, the signal level variation becomes larger, and if the switching occurs during bar code scanning, the switching action may lead to ambiguity in determining whether a received bar code signal corresponds to a “bar” or “no-bar” condition within the bar code being scanned.
A low-pass filter (LPF) may be used to resolve the signal interpretation issue discussed above. However, it is difficult to ensure that the LPF is tuned with sufficient precision to ensure accurate interpretation of signals that are received as the resistance values are in the midst of being altered. Accordingly, there is a need in the art to ensure that ambiguity in interpreting signal values from a bar code reader is not generated by altering the gain of an amplification circuit while reading a bar code.